Re: [PATCH v5 3/7] cxl/acpi: Add background worker to coordinate with cxl_mem probe completion

["Koralahalli Channabasappa, Smita" <Smita.KoralahalliChannabasappa@amd.com>]

Hi Zhijian,

On 8/26/2025 11:30 PM, Zhijian Li (Fujitsu) wrote:
> All,
> 
> 
> I have confirmed that in the !CXL_REGION configuration, the same environment may fail to fall back to hmem.(Your new patch cannot resolve this issue)
> 
> In my environment:
> - There are two CXL memory devices corresponding to:
>     ```
>     5d0000000-6cffffff : CXL Window 0
>     6d0000000-7cffffff : CXL Window 1
>     ```
> - E820 table contains a 'soft reserved' entry:
>     ```
>     [    0.000000] BIOS-e820: [mem 0x00000005d0000000-0x00000007cfffffff] soft reserved
>     ```
> 
> However, since my ACPI SRAT doesn't describe the CXL memory devices (the point), `acpi/hmat.c` won't allocate memory targets for them. This prevents the call chain:
> ```c
> hmat_register_target_devices() // for each SRAT-described target
>     -> hmem_register_resource()
>       -> insert entry into "HMEM devices" resource
> ```
> 
> Therefore, for successful fallback to hmem in this environment: `dax_hmem.ko` and `kmem.ko` must request resources BEFORE `cxl_acpi.ko` inserts 'CXL Window X'
> 
> However the kernel cannot guarantee this initialization order.
> 
> When cxl_acpi runs before dax_kmem/kmem:
> ```
> (built-in)                 CXL_REGION=n
> driver/dax/hmem/device.c  cxl_acpi.ko      dax_hmem.ko               kmem.ko
> 
> (1) Add entry '15d0000000-7cfffffff'
>                                            (2) Traverse "HMEM devices"
>                                                Insert to iomem:
>                                                5d0000000-7cffffff : Soft Reserved
> 
>                        (3) Insert CXL Window 0/1
>                            /proc/iomem shows:
>                            5d0000000-7cffffff : Soft Reserved
>                              5d0000000-6cffffff : CXL Window 0
>                              6d0000000-7cffffff : CXL Window 1
> 
>                                           (4) Create dax device
>                                                                   (5) request_mem_region() fails
>                                                                     for 5d0000000-7cffffff
>                                                                     Reason: Children of 'Soft Reserved'
>                                                                     (CXL Windows 0/1) don't cover full range
> ```
> 

Thanks for confirming the failure point. I was thinking of two possible 
ways forward here, and I would like to get feedback from others:

[1] Teach dax_hmem to split when the parent claim fails:
If __request_region() fails for the top-level Soft Reserved range 
because IORES_DESC_CXL children already exist, dax_hmem could iterate 
those windows and register each one individually. The downside is that 
it adds some complexity and feels a bit like papering over the fact that 
CXL should eventually own all of this memory. As Dan mentioned, the 
long-term plan is for Linux to not need the soft-reserve fallback at 
all, and simply ignore Soft Reserve for CXL Windows because the CXL 
subsystem will handle it.

[2] Always unconditionally load CXL early..
Call request_module("cxl_acpi"); request_module("cxl_pci"); from 
dax_hmem_init() (without the IS_ENABLED(CONFIG_DEV_DAX_CXL) guard). If 
those are y/m, they’ll be present; if n, it’s a no-op. Then in 
hmem_register_device() drop the IS_ENABLED(CONFIG_DEV_DAX_CXL) gate and do:

if (region_intersects(res->start, resource_size(res),
                       IORESOURCE_MEM, IORES_DESC_CXL) !=REGION_DISJOINT)
	/* defer to CXL */;

and defer to CXL if windows are present. This makes Soft Reserved 
unavailable once CXL Windows have been discovered, even if CXL_REGION is 
disabled. That aligns better with the idea that “CXL should win” 
whenever a window is visible (This also needs to be considered alongside 
patch 6/6 in my series.)

With CXL_REGION=n there would be no devdax and no kmem for that range; 
proc/iomem would show only the windows something like below

850000000-284fffffff : CXL Window 0
2850000000-484fffffff : CXL Window 1
4850000000-684fffffff : CXL Window 2

That means the memory is left unclaimed/unavailable.. (no System RAM, no 
/dev/dax). Is that acceptable when CXL_REGION is disabled?

Thanks
Smita
> ---------------------
> In my another environment where ACPI SRAT has separate entries per CXL device:
> 1. `acpi/hmat.c` inserts two entries into "HMEM devices":
>      - 5d0000000-6cffffff
>      - 6d0000000-7cffffff
> 
> 2. Regardless of module order, dax/kmem requests per-device resources, resulting in:
>      ```
>      5d0000000-7cffffff : Soft Reserved
>          5d0000000-6cffffff : CXL Window 0
>              5d0000000-6cffffff : dax0.0
>                  5d0000000-6cffffff : System RAM (kmem)
>          6d0000000-7cffffff : CXL Window 1
>              6d0000000-7cffffff : dax1.0
>                  6d0000000-7cffffff : System RAM (kmem)
>      ```
> 
> Thanks,
> Zhijian
> 
> 
> On 25/08/2025 15:50, Li Zhijian wrote:
>>
>>
>> On 22/08/2025 11:56, Koralahalli Channabasappa, Smita wrote:
>>>>
>>>>>
>>>>>>        ```
>>>>>>
>>>>>> 3. When CXL_REGION is disabled, there is a failure to fallback to dax_hmem, in which case only CXL Window X is visible.
>>>>>
>>>>> Haven't tested !CXL_REGION yet.
>>>
>>> When CXL_REGION is disabled, DEV_DAX_CXL will also be disabled. So dax_hmem should handle it.
>>
>> Yes, falling back to dax_hmem/kmem is the result we expect.
>> I haven't figured out the root cause of the issue yet, but I can tell you that in my QEMU environment,
>> there is currently a certain probability that it cannot fall back to dax_hmem/kmem.
>>
>> Upon its failure, I observed the following warnings and errors (with my local fixup kernel).
>> [   12.203254] kmem dax0.0: mapping0: 0x5d0000000-0x7cfffffff could not reserve region
>> [   12.203437] kmem dax0.0: probe with driver kmem failed with error -16
>>
>>
>>
>>> I was able to fallback to dax_hmem. But let me know if I'm missing something.
>>>
>>> config DEV_DAX_CXL
>>>           tristate "CXL DAX: direct access to CXL RAM regions"
>>>           depends on CXL_BUS && CXL_REGION && DEV_DAX
>>> ..
>>>
>>>>>
>>>>>>        On failure:
>>>>>>        ```
>>>>>>        100000000-27ffffff : System RAM
>>>>>>        5c0001128-5c00011b7 : port1
>>>>>>        5c0011128-5c00111b7 : port2
>>>>>>        5d0000000-6cffffff : CXL Window 0
>>>>>>        6d0000000-7cffffff : CXL Window 1
>>>>>>        7000000000-700000ffff : PCI Bus 0000:0c
>>>>>>          7000000000-700000ffff : 0000:0c:00.0
>>>>>>            7000001080-70000010d7 : mem1
>>>>>>        ```
>>>>>>
>>>>>>        On success:
>>>>>>        ```
>>>>>>        5d0000000-7cffffff : dax0.0
>>>>>>          5d0000000-7cffffff : System RAM (kmem)
>>>>>>            5d0000000-6cffffff : CXL Window 0
>>>>>>            6d0000000-7cffffff : CXL Window 1
>>>>>>        ```